Transgenic genes

Promiscuous plants could spread genes to weeds

September 2, 1998

Crops engineered to contain genes that give them resistance to pests or the ability to produce lots of seeds could pass these genes to their weedier cousins, producing hybrid strains of super-weeds, says Joy Bergelson, assistant professor of ecology and evolution at the University of Chicago. Her findings will be reported in the September 3, 1998 correspondence pages of Nature.

Artificially created plants, like wild plants, can breed with closely related species to produce hybrids in what is called outcrossing. For example, corn, which is a grass, can cross with timothy grass--an abundant weed. If the corn contains a gene that confers resistance to a pesticide, the resultant "weedy" hybrid may become a pesticide-resistant nuisance that can compete with crops for water and nutrients.

Farmers haven't worried about outcrossing, because most crop plants are self- fertilizing; so their genes were considered unlikely to migrate to other species. But Bergelson has demonstrated that plants thought to be "selfing" can outcross with closely related species, and that the rate of outcrossing appears to be enhanced by the fact that they are transgenic.

To test the frequency of outcrossing in transgenic plants, Bergelson grew three different kinds of Arabidopsis--a selfing mustard plant--and planted them together on a plot in central Illinois. One type contained a point mutation in the Csr1-1 gene, making it resistant to chlorsulfuron--an herbicide. The second variety of Arabidopsis was engineered with a mutated Csr1-1 gene, producing a transgenic chlorsulfuron-resistant plant. The third type were wild-type Arabidopsis with normal copies of the Csr1-1 gene.

The plants were left alone, and their seeds were collected at the season's end. Plants grown from the seeds were tested for resistance to chlorsulfuron. Resistant plants were further tested to determine if they inherited the resistance gene from Crs1-1 mutants or from the transgenic plants containing the implanted Csr1-1 mutant gene.

Bergelson found that 20 times as many resistant progeny received their resistance from the transgenic plants as from the mutant plants.

"It is unclear why the transgenic plants had such an abnormally high incidence of outcrossing," says Bergelson, "but the results demonstrate that genetic engineering can substantially increase the incidence of outcrossing in a selfing species." She warns that the widespread use of transgenic crops may directly cause the creation of weeds with traits intended to increase the fitness of crops, spurring a need for new pesticides.